5. Cladistics

Question 1

Define a ‘clade’.

If you’re interested or it helps you learn it: where is the term ‘clade’ derived from?

Answer 1

a group of organisms that evolved from a common ancestor:

• can be large groups with a common ancestor far back in evolution
• or can be smaller groups with a more recent common ancestor

‘clade’ derived from Ancient Greek “klados” - a branch

Question 2

How can evidence for which species are part of a clade be obtained?

Answer 2

• by looking at any characteristics anatomical features are now rarely used because it is sometimes hard to distinguish between homologous traites

Question 3

Evidence for which species are part of a clade can be obtained by looking at any characteristics. Is there an exception to this statement? Why?

Answer 3

anatomical features are now rarely used:
- because it is sometimes hard to distinguish between homologous traits derived from a common ancestor, and analogous characteristics that have developed by convergent evolution

Question 4

Evidence for which species are part of a clade can be obtained by looking at any characteristics. What is usually used to identify which clade a species should be in? Why?

Answer 4

base sequences of a gene are used, or the corresponding amino acid sequence of a protein:
- sequence differences accumulate gradually, so there is a positive correlation between the number of differences between two species and the time they diverged from a common ancestor

Question 5

Why is classifying humans particularly difficult?

Answer 5

because the differences between humans and other species seem so huge to us

Question 6

Give an example of how can cladistics be used to produce an objective classification of humans?

Answer 6

sequencing mitochondrial DNA and then constructing a cladogram (p.70)

Question 7

How long ago did humans and chimpanzees split into separate classifications (according to sequence differences in mitochondrial DNA)?

Answer 7

5mil years ago

Question 8

How long ago did Africans and European/Japanese split into separate classifications (according to sequence differences in mitochondrial DNA)?

Answer 8

140,000 years ago

Question 9

How long ago did European and Japanese split into separate classifications (according to sequence differences in mitochondrial DNA)?

Answer 9

70,000 years ago

Question 10

What are cladograms?

Answer 10

tree diagrams that show the most probable sequence of divergence in clades

Question 11

What are branching points on cladograms called? What do they show?

Answer 11

• branching points called nodes

- show groups of organisms which are related, and therefore presumably had common ancestry

Question 12

What two things related to cladograms grew towards the end of the 20th century? What is significant about this?

Answer 12

1 amount of base and amino acid sequence data
2 analytical power of computers
grew exponentially.

Cladograms could therefore be produced showing the probable evolutionary relationships of large groups of species.

Question 13

Better (more accurate and larger) cladograms could be made towards the end of the 20th century. What did this lead to?

Answer 13

a re-evaluation of the classification of many groups of organisms

Question 14

What is ‘cladistics’?

Answer 14

the method of classification using cladograms

Question 15

What has evidence from cladistics as a new method of classification shown us?

Answer 15

that classifications of some groups based on structure did not correspond with the evolutionary origins of a group or species

Question 16

Give an example of an organism that has been re-classifed following new evidence generated by cladistics.

Answer 16

the figwort family of plants (Scrophulariaceae)

Question 17

What did cladograms of the figwort family of plants (Scrophulariaceae) reveal?

Answer 17

• species in the family did not all share a recent common ancestor

Question 18

How did this new evidence - that species in the Scrophulariaceae family did not all share a recent common ancestor - change our classification of Scrophulariaceae?

Answer 18

1 some genera were therefore moved to:

• the plantain and broomrape families
• two newly created families - lindernia and calceolaria families

2 two existing families, the buddleja and myoporum families, were found to contain species that shared common ancestry with the figwort (Scrophulariaceae) family so they were merged with it

Question 19

Is cladistics the definitive way of classifying plants?

Answer 19

no - just as the traditional classification was falsified and replaced as a result of cladistics, evidence may be discovered that shows further reclassification is needed

Question 20

What two things can analysing cladograms show us?

Answer 20

• how closely organisms are related to each other

- the probable sequence in which groups split

Question 21

Practice reading cladograms.

Answer 21

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